.gitignore update for generated files in _site/grunt
.gitignore update for generated files in _site/grunt

image-tester/web-toolkit,image-tester/web-toolkit,image-tester/web-toolkit,skyglobal/web-toolkit,aldreth/web-toolkit,aldreth/web-toolkit,aldreth/web-toolkit,skyglobal/web-toolkit,skyglobal/web-toolkit